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Re: A bewitched voltage divider?

 

A little comic relief- Because the circuit is "Bewitched", try doing the nose twitch that Samantha does. Who knows, it might help?!? :)

??? Brenda Gentry, KA2IVY

On 2/20/25 21:25, ed breya wrote:
The noted symptom on-screen is consistent with the divider giving too high an output result - this causes the HV to drop, so the display blooms oversize. Also, that "spikey and noisey" TP condition during the fault is probably what you'd expect when the regulation loop goes open from loss of feedback or something hitting a limit. The problem I think is that something is wrong with one of the parts, causing intermittent jumping of the apparent divide ratio. It can be one of the resistors' elements or connections going bad, or a bad solder joint, or the mux.

Remember the 4051 is a bunch of CMOS transmission gates that are actuated by changing gate voltages internally. Being leaky can cause different troubles depending on the power supplies and signals used. If it's a fully bipolar switching arrangement, a fault leakage current may be of either polarity, and would actually cause an offset, not a scale change - but you don't know yet until getting some measurements. Since the signal is negative, the Vss would have to be negative and bigger, and there's probably a plus Vdd analog supply and plus 5V logic (or one in the same), so faults could go either way. I'd recommend again that if you have not eliminated the 4051, to do so and be sure.

I'd still suspect possible resistor or contact oddities, but since you've done a lot already to eliminate those, that leaves I think only the mux, or maybe something downstream from that. The CMOS parts have pretty good transient voltage protection, but they can still randomly fail, and that signal is part of a HV environment, and you don't know what sorts of things may have happened in there before. Good luck.

Ed
Re: A bewitched voltage divider?

 

The noted symptom on-screen is consistent with the divider giving too high an output result - this causes the HV to drop, so the display blooms oversize. Also, that "spikey and noisey" TP condition during the fault is probably what you'd expect when the regulation loop goes open from loss of feedback or something hitting a limit. The problem I think is that something is wrong with one of the parts, causing intermittent jumping of the apparent divide ratio. It can be one of the resistors' elements or connections going bad, or a bad solder joint, or the mux.

Remember the 4051 is a bunch of CMOS transmission gates that are actuated by changing gate voltages internally. Being leaky can cause different troubles depending on the power supplies and signals used. If it's a fully bipolar switching arrangement, a fault leakage current may be of either polarity, and would actually cause an offset, not a scale change - but you don't know yet until getting some measurements. Since the signal is negative, the Vss would have to be negative and bigger, and there's probably a plus Vdd analog supply and plus 5V logic (or one in the same), so faults could go either way. I'd recommend again that if you have not eliminated the 4051, to do so and be sure.

I'd still suspect possible resistor or contact oddities, but since you've done a lot already to eliminate those, that leaves I think only the mux, or maybe something downstream from that. The CMOS parts have pretty good transient voltage protection, but they can still randomly fail, and that signal is part of a HV environment, and you don't know what sorts of things may have happened in there before. Good luck.

Ed
Re: Help! 2465A CRT Focus Troubles

 

Rusty,

R1853, R1854, R1871, R1872 and R1880 are known problems. Replace them with 1W 1% types. When any of these go bad, focus will not work. R1991 should be replaced with a Vishay or Ohmite type.

Mark
Re: A bewitched voltage divider?

 

Hi guys and thanks for your help. Here is a small update:
- TESTS - I disassembled the board, removed the critical components and checked them. I have three meters capable of measuring 10M. Two say the value is as it should be, one says +200K. I cleaned everything carefully and reassembled everything. I am now convinced that the components are OK. By the way, as far as I know, resistors increase in value over the years. In this case they should have changed a lot towards lower resistance. Also, if the 4051 was leaking, I would read a lower voltage, not higher.
- ANOMALY - Using R1355, I can easily set 1,600V on pin 4 of U1540, but I cannot go beyond 1,550V measured with the HV probe. This makes me think that, as you say, the problem could be in the HV probe measurement.
- ANOMALY? - That could be the point. Looking at TP1250, I see a 200mV oscillation at about 36 kHz, with a DC value of about -2.8V. From this, I can imagine that the -1600V has a significant 36 kHz ripple, and perhaps this could also be misleading the HV probe. This AC component is filtered by C1420, so the 1600V at the test point is accurate and very stable, but how can the HV probe behave in this case?

SO MY QUESTION is: should the cathode voltage be perfectly levelled? If so, my 11302 is faulty, otherwise I am looking for a problem that does not exist.

I forgot to say why I am checking all this. The 11302 seems to be working fine, but from time to time when I turn it on, the display is completely defocused and bigger than it should be. I turn it off and on, and most of the time the problem goes away. I could see that when this happens, the signal on the TP1250 becomes "spiky and noisy" as if the inverter has gone crazy (I upload a picture: on the left = inverter crazy; on the right = all OK).

Thanks again
Gianni
Re: A bewitched voltage divider?

 

Gianni,

Ed B. has a good idea to isolate the divider from the mux. As you have soldering iron hot and ready to go, I suggest that you lift the resistors R1422 and R1100 at the -1.6V junction such that the 4051 mux (U1540) is not connected to the divider. Re-connect the resistors with a jumper wire and test the -1.6V point (low point of the divider).

If you need help to navigate the layout to determine which resistor ends to lift, I can post a layout map for the 11302A. I don't have the 11302 layout. Compared to the 11302, there were no significant changes in that area; so, the components should be in the same locations.

I am assuming that you have indeed verified that you have -1600V at the high end of the divider.

Clark
Re: A bewitched voltage divider?

 

On 20 Feb 2025 9:29 am, Robert Gibson wrote:

Have you verified the accuracy of the HV probe and "VOM (or whatever meter type in use)?" The resistors there may have drifted.
Robert has an excellent point. Instruments and test equipment need to have their accuracy checked periodically. I only have one instrument which goes to the cal lab for an annual calibration. It is my 4-1/2 digit, true RMS. Keithley 179 bench DMM. I also own a Leo Bodnar GPS frequency standard. Those are my standards for checking the rest of my shop and field instruments and test equipment.

An HV probe is intended to be used with a meter of specified input resistance, and the probe will only meet the accuracy specs with the specified input resistance. That input resistance is part of a voltage divider. A different input resistance reduces the accuracy of the HV probe, and unless you perform the calculation to compensate for the different meter resistance your measurements will not be accurate.

It only takes a minute amount of contamination to ruin an HV probe. Handling the probe resistor with bare fingers is often enough to ruin that resistor.
--
Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
Re: Help! 2465A CRT Focus Troubles

 

Bonjour WORGH, have used,and fixed 2465/7/B since 1990s.


I am very wary of any older SN, or no suffex 2465 or A.

Any vendor with "government" is also a flag.

Repair can be PS, or HV, and control board,A5 CAL,RAM, SMD,lytic corrosion

CRT damage is rare, unless it,was dropped or mispackaed in shipment

The very,severe defects appears more,than a PS, or just bad HV.

Even for very experienced 2465 veterans, with best CAL equipment, it can take hours, days weeks...

One,dog I have had on the bench since 2018!

You should get the full service manual for the model and serial, follow the excellent flow charts and check each board and subassembly in sequence

Alternatively just write it off as,a parts,donor, and look out for a,100% working 2465B.

Bon courage


Jon
Re: A bewitched voltage divider?

 

Is it still connected to the 4051 during measurements? Try without any other connections and of course be aware of the DVM load effects. If the 4051 is leaky, it could cause grief. Also try getting a reading from the output of the mux while the divider output is shorted to ground.

Ed
Help! 2465A CRT Focus Troubles

 

Hello

I am at a standstill with figuring out why the CRT of my 2465A is blurry and won't focus.

I purchased this unit from Government Planet last summer with the idea that I would probably have to troubleshoot it. I turned it on and it made a screeching sound and had no display. I thought the sound was coming from the fan but no it was the high voltage board. Checked the voltages on J119 and they were all good with the exception of the -15V rail which had high ripple. Decided to recap the LVPS with a kit from Condor Audio. This fixed the -15V rail. Turned it on and there was a blurry dispay but intermittent screeching from the high voltage board. Looked closer at this and discovered that the ceramic of T1970 was cracked in several places. Purchased a new high voltage board from QService and installed it. Now no screeching but the display remains blurry with the Read Out letters wider and taller than they should be and the same for the traces. I have uploaded 2 pictures to the file Amberwave 2465a that shows what I'm talking about. I checked voltages to the CRT socket at pins 8(31V), 12(-108V to -186V) and 13(-192V to -224V) and they look reasonable. DAC reference is right at 10V. When I input a signal into the scope I can just make out that it is a sign wave.

I just don't know where to go from here. I have invested a lot of time and dollars in this scope to just give up. It would seem that this isn't a HV board issue as both boards gave a blurry image. Could this be the CRT tube itself or one of the IC's? I would really appreciate some guidance on where to go from here from someone more experienced with these scopes than me

Rusty Hilliard
W0RGH
Re: A bewitched voltage divider?

 

Gianni,

According to the schematic, cathode voltage is supposed to be -1600 V. At 1000:1 divider would give 1.60 V, which it does. You say the input is -1510 V. Have you verified the accuracy of the HV probe and "VOM (or whatever meter type in use)?" The resistors there may have drifted.

--Robert

On 2/20/2025 2:49 AM, Gianni Becattini via groups.io wrote:
Hi Guys,
I am trying to fix some problems on my 11302, but I am stuck on a problem that seems very simple, but which I cannot overcome: a simple voltage divider. I am probably doing something wrong, but I cannot guess what (I added an album among the photos with the simplified schematic diagram).

The 11302 "reads" the CRT cathode voltage using a voltage divider, selected by a 4051 analog multiplexer. It should be simple: the voltage divider is made up of a 20 M ohm and a 20 kohm resistor (the former is actually 2 x 10 Mohm resistors). So the output should be one volt for every 1000 volts in the input (well, 0.999).
For me this is not the case: the divider output is significantly higher than it should be (with -1.6 V in the output, the input is actually -1.510 V).

Easy, you say, one of the resistors has changed its value over time, but it is not so. I desoldered one extreme of them and they are all very precise. Another hypothesis was that the CRT mesh was causing some problem, but it is not: I disconnected it and nothing changed. I neglected the C1100 and C1420 capacitors because they could only lower the voltage, not make it greater.

The last thing I didn't check yet is the dirt on the lower part of the board, but before disassembling it I wanted to hear your suggestions, if you like.

I will be very grateful for any help, I am getting crazy. Thanks Gianni
Re: A bewitched voltage divider?

 

On Thu, Feb 20, 2025 at 4:49?AM Gianni Becattini via groups.io
<Giovanni.becattini@...> wrote:

I will be very grateful for any help, I am getting crazy. Thanks Gianni
This is how I felt until I discovered that my 485 had a voltage dependent
"precision" resistor in the circuit that derives 13V from the 50V rail. See
/g/TekScopes/message/113493. I have no idea why this
happened, nor how common it is, but apparently it can happen.
In your case I guess it could be as simple as some kind of contamination on
or around the 10M resistors, which doesn't affect your DMM measurements,
but appears at the working voltage?
Re: A bewitched voltage divider?

 

I add a question: the cathode voltage is supposed to have some ripple or to be steady? When I measure the HV, I use the HV-probe with the VOM but I could understand that the voltage does have a ripple, looking at TP1250.
A bewitched voltage divider?

 

Hi Guys,
I am trying to fix some problems on my 11302, but I am stuck on a problem that seems very simple, but which I cannot overcome: a simple voltage divider. I am probably doing something wrong, but I cannot guess what (I added an album among the photos with the simplified schematic diagram).

The 11302 "reads" the CRT cathode voltage using a voltage divider, selected by a 4051 analog multiplexer. It should be simple: the voltage divider is made up of a 20 M ohm and a 20 kohm resistor (the former is actually 2 x 10 Mohm resistors). So the output should be one volt for every 1000 volts in the input (well, 0.999).
For me this is not the case: the divider output is significantly higher than it should be (with -1.6 V in the output, the input is actually -1.510 V).

Easy, you say, one of the resistors has changed its value over time, but it is not so. I desoldered one extreme of them and they are all very precise. Another hypothesis was that the CRT mesh was causing some problem, but it is not: I disconnected it and nothing changed. I neglected the C1100 and C1420 capacitors because they could only lower the voltage, not make it greater.

The last thing I didn't check yet is the dirt on the lower part of the board, but before disassembling it I wanted to hear your suggestions, if you like.

I will be very grateful for any help, I am getting crazy. Thanks Gianni
Added photo album Bewitched voltage divider (11302) #photo-notice

Group Notification
 

Gianni Becattini <Giovanni.becattini@...> added the photo album Bewitched voltage divider (11302) ( /g/TekScopes/album?id=300803 ) : see my emailò
Re: Tektronix 145 PAL test signal generator - repair (oven control circuit and maybe more)

 

Hi all

Thanks for your inputs... yes it's mostly for preservation reasons... how many people or institutions are left with a functional Tektronix 145? I initially picked it up for spares but it's so well made it would be sad to trash it if repair is possible. If I can't get the crystal oven to work or other BER situation I can still gut it.

I do have old PAL televisions to repair/restore so it could also theoretically serve a purpose.

I'm not sure replacing the resistor will help with anything.. it reads within spec, is °C 130~ too hot for a larger resistor? I checked all the socketed transistors and they're OK. The thermistor is not open, though I'm not sure about its value (I should probably remove it out of circuit to check). I will have a look into the power supply section and try to get it within spec before I continue.

After 30 mins of waiting, the TP1100 voltage stopped at 3.95 V (the manual says it should come on before 20 mins - the recommended warmup time). Maybe it does need the PSU section dialed in.
Re: Extender Card for TM500/TM5000 with Test points on every line.

 

I have my boards made at PCBWay. They produce extremely high quality boards at very reasonable prices. When gold fingers are involved using the traditional gold over nickel the fingers are electroplated which takes extensive time and Gold is EXPENSIVE. The difference in cost between the 5 board minimum and 25 boards is only about $35, once you cover the extra setup. If I pursue the project, they will also do a 30 degree bevel on the finger end of the board to allow it to fit into the female connector easily. I am investigating a less costly but still rugged finger treatment called Enepig (electroless nickel electroless palladium immersion gold). This finish is cheaper to apply than the the electroplated gold over nickel, but still make a set of boards in the $150 + range instead of the $20+ range.

Another expensive item is the Female connector. I plan to use the Sullins ECM28DRKN-S!3 which can be ordered from Digikey for $10.72 each, but minimum order is 10 pcs.
This very high quality connector uses full bellows Gold over Nickel phosphor bronze contacts. The contacts and the plastic are rated at 125 degrees C.
The gold plated 4-pin headers are not cheap, either and 56 of them are required for each extender board. FCI part number 95293--101-04LH (30u" gold over NI) or 95283-801LF (15u" Gold over NI). The Jumpers are also expensive: FCI 65474-001. 56 required, 60 recommended in case you lose a couple.

The design is very high quality, but as a result it is not cheap. Actually the construction will be superior to the male end of my Tek 067-0645-02 TM500 extender cable. The male end of their cable did not have the 30 degree bevel which makes it much harder to insert into a female connector than the typical plug in.

I am continuing to research the costs for this project, but it is not going to come in at the $50 price of the one on eBay that uses raw copper for the board and fingers, with no test points.

Hopefully there will be enough interest to group buy the parts.

Steve

On 2025-02-18 10:22, Roy Thistle via groups.io wrote:
On Mon, Feb 17, 2025 at 12:56 PM, Eric Garner wrote:

Not to dissuade OP from their project
Well... many of us...on this forum... IMO... are hobbyists.
Probably, I am not very smart... but AFAICT... time, connectors, and
fiddling around with shipping, can be expensive... but, high quality
PCB are not.
On way around the distribution problem is to register a 'project' with
a PCB manufacturer, and then anyone can order the PCB.
Also, at least down to common components, the PCB manufacturer can
also manufacture the thing to that level, and even provide a kit.
The connectors for these...ARAIK... are not as much of a problem
sourcing anymore.
If one is worried about quality, and not their wallet... then buying
the Tektronix ones... when they show up... might be a better solution.
Re: Tektronix 145 PAL test signal generator - repair (oven control circuit and maybe more)

 

Luca,

R1156 is too hot. Replace with a 5W. Look around the TP1100 for bad parts. The thermistor could be open. Dirty pins/sockets could be the problem. Check the ripple in the power supply. At the age of this unit, recapping may be necessary.

Mark
Re: Tektronix 145 PAL test signal generator - repair (oven control circuit and maybe more)

 

To fix PAL standard televisions?

Yes, people are into preserving antique televisions and equipment.







Now if 145 PAL does not refer to test equipment for the non-NTSC standard televisions them I am out.

Frank DuVal

On 2/19/2025 11:44 AM, EB4APL via groups.io wrote:
Excuse me, but why you would want a functional PAL signal generator?

El 19/02/2025 a las 17:31, Luca via groups.io escribió:
Hi all

Here /g/TekScopes/topic/107425848 H?kan kindly provided the service manual for this signal generator.

I had time to have a look inside and found a shorted cap on the -15V supply (C1200 of A13 board), now the power supply seems to behave a bit better (the 3.6 V rail is still off, at 3.3~ ish Volts)

According to the manual the oven temp normal light should come on but this never happens (this circuit doesn't seem to use the 3.6V rail so I ignored it for now)

The manual says that TP1100 should be around 4.1-4.2 Volts for the "oven temp normal" light to come on but that never happens.. it hovers around 3.5 V and varies a bit. I noticed that R1156 gets really hot (>100 °C) but I don't know if this is a fault or not.
I took the oven apart to check the thermal fuse but it seems ok... so I'm not really sure where to go from here.

I updated the album: /g/TekScopes/album?id=296482

Can I plug the oven in without enclosure for testing?

Disclaimer: I obviously have no idea what I'm doing... there doesn't appear to be any notable occurences of this SG on the net so I will try to find faulty or out of spec parts and hope any of you have a good idea where to start

Thanks
Luca
Re: Tektronix 145 PAL test signal generator - repair (oven control circuit and maybe more)

 

Excuse me, but why you would want a functional PAL signal generator?

El 19/02/2025 a las 17:31, Luca via groups.io escribió:
Hi all

Here /g/TekScopes/topic/107425848 H?kan kindly provided the service manual for this signal generator.

I had time to have a look inside and found a shorted cap on the -15V supply (C1200 of A13 board), now the power supply seems to behave a bit better (the 3.6 V rail is still off, at 3.3~ ish Volts)

According to the manual the oven temp normal light should come on but this never happens (this circuit doesn't seem to use the 3.6V rail so I ignored it for now)

The manual says that TP1100 should be around 4.1-4.2 Volts for the "oven temp normal" light to come on but that never happens.. it hovers around 3.5 V and varies a bit. I noticed that R1156 gets really hot (>100 °C) but I don't know if this is a fault or not.
I took the oven apart to check the thermal fuse but it seems ok... so I'm not really sure where to go from here.

I updated the album: /g/TekScopes/album?id=296482

Can I plug the oven in without enclosure for testing?

Disclaimer: I obviously have no idea what I'm doing... there doesn't appear to be any notable occurences of this SG on the net so I will try to find faulty or out of spec parts and hope any of you have a good idea where to start

Thanks
Luca

--
Este correo electrónico ha sido analizado en busca de virus por el software antivirus de Avast.
www.avast.com
Tektronix 145 PAL test signal generator - repair (oven control circuit and maybe more)

 

Hi all

Here /g/TekScopes/topic/107425848 H?kan kindly provided the service manual for this signal generator.

I had time to have a look inside and found a shorted cap on the -15V supply (C1200 of A13 board), now the power supply seems to behave a bit better (the 3.6 V rail is still off, at 3.3~ ish Volts)

According to the manual the oven temp normal light should come on but this never happens (this circuit doesn't seem to use the 3.6V rail so I ignored it for now)

The manual says that TP1100 should be around 4.1-4.2 Volts for the "oven temp normal" light to come on but that never happens.. it hovers around 3.5 V and varies a bit. I noticed that R1156 gets really hot (>100 °C) but I don't know if this is a fault or not.
I took the oven apart to check the thermal fuse but it seems ok... so I'm not really sure where to go from here.

I updated the album: /g/TekScopes/album?id=296482

Can I plug the oven in without enclosure for testing?

Disclaimer: I obviously have no idea what I'm doing... there doesn't appear to be any notable occurences of this SG on the net so I will try to find faulty or out of spec parts and hope any of you have a good idea where to start

Thanks
Luca
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